Aerostatic-lift aircrafts are known (commonly called airships, or dirigibles), which comprise a horizontal tube filled with lighter-than-air gas, such as helium, with associated propelling means and direction-control means. Airships, depending on size, may be provided with a pilot cabin to accomodate a crew, and possibly with a passenger compartment and/or with a freight bay.
Airships have attracted some interest in the past because of their low-power requirements, allowing for unsophisticated engines, of their ability to take-off and land without a need for a dedicated airstrip, and also because they can remain airborne even in case of engine breakdown. Consequently, there have been attempts to broaden the use of airships to different applications, such as terrain-surveillance missions, reconnaissance, freight-and-passenger transport, and other. On the other hand, airships are also regarded as a safe and reliable aircraft, because, due to the volumes and to the limited pressures of their inflated tube, the airship, even in case of tube puncturing, is able to remain airborne for several hours, or at least to descend slowly and without risks.
In spite of the above considerations, the use of airships has been limited until now, in contrast to the more widespread aerodynamic-lift aircrafts, such as aeroplanes and helicopters, mainly because of their low cruising speed and consequent low yeld, and because of their difficult control during hoevering, particularly in case of air currents, however weak, or, even worse, in case of wind gusts.
Another drawback of the conventional airships, as known to persons skilled in the field, is that their mooring is awkward, particularly before the freight, equipment and staff have been loaded. In fact, the airship is suitably sized in relation to the limit load it is meant to carry, i.e., it must be able to take-off even under full-load conditions. By contrast, under unloaded conditions, the aerostatic thrust is excessive because only opposed by the empty weight of the airship, so that the latter requires sturdy mooring structures to be kept down on the ground, which structures, however, are often difficult to locate.
A further drawback of the conventional airships is that, due to their cylindrical profile, they are intrinsically unbalanced and require tail planes to maintain a level trim in flight.
However, one of the most frequent inconveniences with the airships is the loss of the rigid tail planes, because their connection to the nonrigid tube is very awkward, which circumstance causes the airship to become completely uncontrollable and to tilt to a transversal arrangement, with rise of aerodynamic forces and consequent structural collapse.
A further drawback of the airships is they are very sizable compared with their carrying capacity, so that, with heavy loads, huge aircraft must be used.
On the other hand, the conventional aerodynamic-lift aeroplanes have drawbacks as well, such as that they need a sophisticated air-traffic control system, as well as long airstrips for taking-off and landing at organized terminals, which, however, have high equipping-and-mainteinance costs.
Furthermore, in order to provide a reasonable degree of safety with any flight condition, airplanes require redundancy of engines and flight systems and, in case of engine breakdown, they must suspend the mission and land to the nearest airport in conditions of priority.
Of course, the above drawback is even more serious with the helicopters because, in case of engine breakdown, only attempts may be made to damp the crash by means of a complex auto-rotation maneuver, with uncertain results.
A further drawback of all the conventional aerodynamic-lift aircrafts is the high-fuel consumption and, consequently, the limited endurance because, though the enough high efficiency, high power is required to fly.